The hypothesis being tested is that psoriasis is a disease mediated by activated (CD25+) T-lymphocytes and that blockage of the IL-2 receptor alpha-subunit (CD25 moiety) will decrease lymphocyte activation and thus improve psoriasis. Patients meeting inclusion criteria will be enrolled into this study. Following enrollment the following will be done: physical examination; photography of skin lesions; psoriasis severity assessment score; biopsy of unaffected and lesional skin; baseline laboratory evaluations, including cbc, lymphocyte subset analysis, serum chemistry, HIV, serologies for hepatitis B & C, beta-BCG (where indicated) and urinalysis; [optional evaluation] ultrasound imaging of psoriatic plaque. Humanized anti-Tac antibodies (anti-Tac-H) will be given as a 30 minute intravenous infusion on the following schedule: day 1 (2 mg/kg); day 15 (1 mg/kg); day 29 (1 mg/kg); day 43 (1 mg/kg). Psoriasis severity will be assessed prior to each infusion with anti-Tac- H and also upon study day 57 (8 weeks after starting treatment). At each treatment, blood samples will be taken before infusion at 30 minutes following the completion of the infusion in order to quantify binding of anti-Tac-H to CD25 molecules on CD4+ and CD8+ lymphocyte subsets. At each of these assessment points, a biopsy will be taken of lesional skin inorder to phenotype lymphocytes subsets in diseased tissues. Following assessment on day 57, patients showing good clinical responses will be re-assessed at 2-4- week intervals to establish durability of any benefit seen. When recurrence or worsening of psoriasis is noted in this follow- up phase, retreatment with a single dose of anti-Tac-H (1 mg/kg) will be permitted to determine its potential to suppress a disease recurrence.